Asphalt plug emplacement process

ABSTRACT

A process for plugging a subterranean earth formation by injecting an asphalt-containing emulsion is improved by a cationic emulsifier and a pH increasing reactant that induces the breaking of the emulsion in response to a time-temperature exposure of selected extent.

This is a division of application Ser. No. 497,011, filed Aug. 13, 1974,now U.S. Pat. No. 3,901,316.

BACKGROUND OF THE INVENTION

The invention relates to a process for decreasing the permeability of apermeable subterranean earth formation. More particularly, it relates toplugging pores located within the first few inches of the portion of anearth formation that is encountered by the bore hole of a well, andprovides a means for doing so in earth formations that are relativelyhighly permeable and/or fractured.

The positioning of a plugging material within the first few inches of anearth formation is particularly valuable for use in the injectivityprofile improving process of the copending patent application by J. H.Thommer, Ser. No. 362,624, filed May 21, 1973, now U.S. Pat. No.3,830,299. In that process, variation with depth in the fluid flowpattern of a well is determined, the first few inches of all permeableportions of the surrounding earth formations are plugged, and then atleast some of the plugged portions are perforated with openings that aresized and located to provide selected rates of flow at selected depths.

Both the utility of plugging a subterranean earth formation and variousways of doing it with an asphalt emulsion were previously known. In 1933the application for U.S. Pat. No. 2,201,459 described the use of aqueousdispersions of asphalt in an aqueous liquid which contained an acidsoap-type emulsifying agent and a pH reducing reactant, such as methylformate to cause the emulsion to break. The T. K. Miles U.S. Pat. No.2,378,235 describes the relatively incomplete breaking of such emulsionsand a procedure for improving that problem, by dissolving a cationicsurfactant in the asphalt so that it migrates to the surface of thedroplets and subsequently causes their coagulation. U.S. Pat. No.3,026,266 describes asphalt emulsions in which the tendency of theasphalt to stick to siliceous materials is improved by using cationicsurfactants as emulsifiers and polyamide condensation products ofpolymerized acids and polyamines as modifiers. U.S. Pat. No. 3,159,976describes an earth formation plugging process for avoiding the slowbreaking tendancy of such cationic asphalt emulsions, by separatelyinjecting the emulsion and a strong base to mix in or near the zone tobe treated and cause a quick coalescence of the asphalt particles. TheH. J. Sommer and W. C. Simpson U.S. Pat. No. 3,324,041 describes apolyepoxide-containing asphalt emulsion in which emulsions of both apolyepoxide and a nonionic surfactant are mixed to form a compositionwhich is subsequently solidified by an interaction of the polyamide andthe polyepoxide.

SUMMARY OF THE INVENTION

The present invention relates to an improved process for plugging apermeable earth formation by injecting into it an oil-in-water emulsionin which asphalt is dissolved or suspended in the oil-phase and theemulsifier is, at least predominately, a cationic surfactant. Thecomposition and concentration of the asphalt-containing oil-phase andthe cationic emulsifier are arranged so that, in the emulsion, thesuspended particles (droplets and/or solids) are sufficiently small andwell dispersed to flow through the pores of the earth formation to betreated, the pH is relatively near neutral, and the emulsion breaks whenthe pH is increased. A pH increasing reactant is dissolved in theaqueous phase of the emulsion and its composition and concentration arecorrelated with the properties and location of the earth formation to betreated so that the pH of the emulsion is increased to one at which theemulsion breaks after the emulsion has at least substantially reachedthe earth formation to be treated.

DESCRIPTION OF THE DRAWING

FIG. 1 shows a plot of the variation with time in the pH of the cationicemulsion containing a pH increasing reactant.

FIG. 2 shows a plot of the variation with temperature of the duration ofthe stability of cationic emulsions containing pH increasing reactant.

DESCRIPTION OF THE INVENTION

The present invention is, at least in part, premised on the following.As the pH of a cationic emulsion is increased from a near neutral valueat which it is stable to a value in or near the basic range, theemulsion becomes destabilized and breaks. The pH at which the breakoccurs is influenced by the composition and concentration of theemulsifier and the asphalt-containing oil phase of the emulsion. Forexample, a surface-active quaternary ammonium salt may retain itscationic activity to a pH from about 8 or 9, while a surfactant salt ofa diamine may lose its protons (and thus lose its cationic emulsifyingcapability) at a pH of around 4 or 5. The stability of a cationicasphalt emulsion can be affected by the action of a pH increasingreactant in a manner such that the reactant can be used to break theemulsion in response to a time-temperature exposure that is attainablein flowing an emulsion from a surface location at which it is compoundedto a subsurface permeable earth formation into which the emulsion isinjected. And, when a cationic asphalt emulsion is caused to break by anincrease of its pH, the asphalt particles tend to agglomerate and clingto each other and the walls of pores or fractures within an earthformation. When an emulsion is being injected into an earth formation tobe plugged, it is preferable that the emulsion break after it has atleast substantially reached the earth formation to be treated. Anearlier breaking tends to plug only the face of the formation, ratherthan forming a plug within the formation. A later breaking tends torequire a longer shut-in time (after the treatment has beensubstantially completed) but does not prevent the formation of aninterval plug.

The pH increasing reactant used in the present process can besubstantially any water-soluble compound or mixture of compounds thatreact in a time and temperature controlled manner and produce a watersoluble material that increases the pH of an aqueous solution. Such areactant may comprise a mixture of a water soluble epoxide, such aspropylene oxide, and a water-soluble chloride, such as sodium chloride;in which case the following reaction occurs: ##STR1## The pH-increasingreactant may comprise a water soluble cyanate, such as sodium cyanate;in which case the following reaction occurs:

    CNO.sup.- + 2H.sup.+ + H.sub.2 O → CO.sub.2 + NH.sub.4 .sup.+

a pH-increasing reactant that is particularly suitable where thetemperature of the earth formation to be treated is above about 160° F,comprises a mixture of a water-soluble nitrite salt, such as sodiumnitrite, and a water soluble amide of carbonic acid, such as urea, inwhich case the following reaction occurs: ##STR2## In general, thepH-increasing reactant can comprise a water soluble amide of carbamic orcarbonic acid such as ammonium carbamate or urea, or their homologs,salts of cyanic acid, such as alkali metal cyanates, cyanamide, etc.

The asphalts (or bituminous materials) used in the present invention canbe substantially any such pyrogeneous distillate or tars composed mainlyof hydrocarbons with small amounts of sulphur, nitrogen, oxygen or thelike substituents. Such materials can include the bituminous materialsdescribed in Abrams Asphalt and Allied Substances, Volume 1, page 57,Fifth Edition. Particularly suitable materials are straight run asphaltshaving penetration values of from about 40 to 300 and softening pointsin the range from about 95° F to 145° F. Asphalt cutbacks such as a 150penetration grade asphalt cut with 5% to 20% diesel oil, or other oilsolvent, are particularly suitable.

It is preferable that the asphalt or asphalt cutback softening point andviscosity be properly selected to enhance the tendency for shallowmatrix plugging. Tests with Berea cores having as much as severalhundred millidarcy permeability indicate that, in order to obtainpenetration of the asphalt into the core and effective plugging, thecutback should be considerably above its softening point at thetreatment temperature. For example, at a treatment temperature of 140°F, a 120 - 150 penetration grade asphalt cutback with 5% by weight ofdiesel oil penetrated a 160 millidarcy Berea core for about 1 inch. Onthe other hand, a similar cutback at a treatment temperature of 70° Fonly penetrated a 129 md Berea core about 1/8 inch. The five percentcutback had a ring and ball softening point of 86° F so that at atreatment temperature of 70° F, the asphalt is below, while at 140° F,it is considerably above its softening point. At 140° F such a cutbackis an effective plugging material having a viscosity of about 100,000centipoise. In general, since the asphalt can be softened by theaddition of an oil solvent (such as diesel oil) after the emulsion hasbeen compounded, it is preferable to start with an emulsion containing arelatively hard asphalt (e.g. one having a softening point likely to benear or above the temperature of the earth formation to be treated) andthen softening the asphalt to the extent required by adding an oilsolvent to the emulsion. The asphalt concentration of the emulsions canvary relatively widely, for example, from about 5 - 75 percent by weightof asphalt. A viscosity convenient for pumping is obtained by having aconcentration in the range of 40 to 65 weight percent asphalt.

The aqueous liquid used in the present invention can be substantiallyany that is compatible with the emulsifier and pH increasing reactant.In general, a relatively low salt content and relatively soft water, ispreferred. A small but significant concentration of a soluble calciumsalt such as calcium chloride is advantageous in increasing thestability of the emulsion (prior to the addition of the pH increasingreactant). Suitable concentrations of calcium chloride range from about0.1 to 0.25 percent by weight of the aqueous liquid. The emulsions ofthe present invention have a good tolerance to dilution with brine. Forexample, the emulsion formulation No. 35 (see Table 1) can be dilutedwith the synthetic brine of Table 2 to provide a stable emulsioncontaining only about 10% hydrocarbon.

The cationic emulsifier can be substantially any surface-active cationicmaterial such as the surface-active salts of amines and quaternaryammonium salts. Suitable surface active cationic emulsifiers arecommercially available, such as: the Arosurf AA emulsifiers from AshlandChemical Company; the Redicote E emulsifiers from Armak ChemicalCompany; the Aliquat fatty quaternary ammonium chlorides, or the Diamfatty diamines from General Mills Chemical Company; the Nalquats,quaternary imidazoline bases, from Melco Chemical Company; the Arquads,alkyl trimethyl ammonium chlorides, from Armak Chemical Co.; the Emcols,substituted triethyl ammonium chlorides from Emulsol Corporation; theRedicote E-5 Emulsifier, or other quaternary ammonium salts such asArosurf AA-22 or AA-57, are particularly preferred. Such quaternaryammonium salts provide cationic emulsions that are (in the absence ofthe pH increasing reactant) relatively stable at near neutral pH's suchas from about 4 to 8.

Where the earth formation to be treated contains relatively large poresand/or fractures, particulate materials, (preferably having sizesranging from relatively very fine particles to particles having sizesequalling at least about 1/3 the effective diameters of the pores to beplugged) can be suspended in the present emulsion. Suitable suspendedparticles can be silica, rubber, carbonate, asphaltic or the likeparticles having sizes ranging from less than about ten microns to morethan about two thousand microns. The capability of the emulsion toretain such solids in suspension can be enhanced by viscosifying theaqueous phase of the emulsion with a water soluble cellulose ether, suchas hydroxyethyl cellulose ether. Particularly suited is the hydrationretarded grade of HEC available from Hercules Chemical as Natrosol 250H.R. This material is easily dispersed at pH's in the range of 2 to 5.Unretarded HEC can be dispersed in the asphalt emulsion by first makinga paste of the HEC in diesel oil. Thickening of the emulsion is greatlyenhanced by increasing the pH to 6.5 to 7.5 after dispersing thepolymer. Granular asphalt gillsonite particles and/or a crumb rubberparticles having a wide range of particles size distribution areparticularly suitable. Such particles are commercially available insizes ranging from 100 microns or less to over 4,000 microns or about3/16 inch in size. A granular asphalt is available from Oil Base, Inc.under the trade name "FormaSeal" or from Halliburton as Gillsonite.Rubber crumbs are available from B. F. Goodrich Co. as Amerpol 1006.Tests with berea cores through which holes having diameters of 1,000microns were drilled indicated that such pores can be plugged to apressure of 1,000 pounds per square inch by using the present type ofasphalt emulsion and a 5050 mixture of granular asphalt and rubbercrumbs to provide a total concentration of 0.5 lbs. solids per gallon ofemulsion.

The initial break time (IBT), i.e., the time at a given temperaturewhich provides a time-temperature exposure that causes the pH increasingreactant to raise the pH of the emulsion to one in which the emulsionbreaks, is dependent upon numerous factors. For emulsions containing agiven pH increasing reactant, the IBT varies with both the compositionand concentration of the asphalt and/or suspended solids. The severityof the time-temperature exposure of a given emulsion depends on thetemperature of the formation to be treated and the rate at which fluidis to be injected into that formation. Where it is desirable to shortenthe IBT time, for example in treating a relatively shallow formation,the pH of the emulsion can be increased by adding one or more watersoluble bases (such as an alkali metal hydroxide, or carbonate, or thelike) until the pH is just slightly below the pH at which the emulsionbreaks. This ensures a relatively quick attainment of the emulsionbreaking pH by the action of the pH increasing reactant. Alternatively,the temperature of the near borehole portion of the earth formationand/or the conduits extending between the emulsion compounding locationand that formation to be treated, can be altered by injecting arelatively hot or cool fluid. When shallow wells are being treated sothat very short IBT's are required (e.g. 5 - 15 minutes), the pH can beincreased to near the point of emulsion instability by the addition ofcaustic just prior to injecting the emulsion. In a well mixed tank, theamount of caustic required can be judged by observing the trail ofasphalt formed as the caustic is poured onto the surface of theemulsion. Alternately, the caustic can be mixed in line either at thewell head or down hole by simultaneous injection of emulsion and causticsolution.

For treatment of "vacuum" wells, determinations can be made of thebottom hole pressure (by means of a downhole pressure gauge ormeasurements of the fluid column in the well, or the like), in order tofollow the plugging process. Where positive surface injection pressureis needed to inject fluid, the emulsion being injected is maintained ata selected monitored moderate injection pressure to ensure that theinjection pressure is kept below the fracturing pressure of theformation. This can be readily accomplished by use of a by-pass flowloop containing a pressure relief valve which is set to open at apressure below the fracturing pressure of the formation. Once the wellhas been plugged to injection at this pressure it must be reperforatedbefore attempting to establish the desired injection rate into the well.

EXAMPLES

A series of laboratory tests were conducted using a cationic asphaltemulsion with a P.O. (propylene oxide) breaker system and bridgingsolids to plug a 0.040-inch or 0.070-inch diameter hole which weredrilled through 1.75- or 2.00-inch long Berea cores. Such a "large pore"through the core perhaps represents the type of permeability to beplugged in a fractured or extremely permeable earth formation.

A cationic asphalt Emulsion No. 35 is described in Table 1. It contains20 percent diesel oil in the hydrocarbon phase. Emulsion No. 35-C is asimilar formulation with the hydrocarbon phase containing only tenpercent diesel oil.

                                      TABLE 1                                     __________________________________________________________________________    CATIONIC ASPHALT EMULSION FORMULATION NO. 35                                  Two percent Armak Chemical Co. E-5 Emulsifier                                                          2   drums E-5                                             (Quaternary Amine)                                                       65 percent Hydrocarbon                                                             120-150 Penetration grade asphalt                                                                 52  drums asphalt                                         cut back with 20 percent diesel oil                                                               13  drums diesel oil                                 35 percent aqueous phase 33  drums water                                                               100 drums total                                      0.5 lb/drum CaCl.sub.2 (0.1 wt. %)                                                                     50  lb CaCl.sub.2                                    __________________________________________________________________________

In the tests in which the asphalt emulsion was allowed to break inresponse to a time-temperature exposure (rather than theslower-occurring adsorption on the rocks), three gals/bbl of propyleneoxide (P.O.) was used as a breaker. At a one bbl/bbl emulsion of brinedilution, the break time at room temperature is about one hour. Thebrine used for dilution or brine flow was the Synthetic Brine describedin Table 2.

                                      TABLE 2                                     __________________________________________________________________________    SYNTHETIC BRINE                                                                                              Cation                                                                        Field Brine                                                         Cation                                                                             Cation                                                                             Analysis                                                                              Anion                                                                              Cl.sup.-                                                                           SO.sub.4 .sup.=              Salt MW   gm/liter                                                                           mole/liter                                                                          MW   mg/l mg/l    MW   mg/l mg/l                         __________________________________________________________________________    NaCl 58.46                                                                              76.098                                                                             1.3017                                                                              23   29,939                                                                             29,941  35.46                                                                              46,154                            CaCl.sub.2                                                                         110.9                                                                              12.088                                                                             0.1090                                                                              40   4,360                                                                              4,360   35.46                                                                              7,730                             MgCl.sub.2                                                                         95.22                                                                              6.088                                                                              0.0639                                                                              24.32                                                                              1,554                                                                              1,555   35.46                                                                              4,531                             FeSO.sub.4                                                                         151.9                                                                              0.057                                                                              0.000375                                                                            55.85                                                                              21   21.1    96.066    36                           TOTALS                    35,874                                                                             35,877       58,415                                                                             36                           Field Brine Analysis      35,877            58,271                                                                             35                           __________________________________________________________________________

The cores used in these tests were Berea sandstone cores withdimensions, one-inch diameter × 1.75- or 2.00-inch length. They werebonded into a five-inch long lucite core holder (which allowedobservation of the fluid at the core face of e.g., emulsion, aqueousphase of broken emulsion, or hydrocarbon phase of broken emulsion). Inthis series of tests, a 0.040-inch diameter or 0.070-inch diameter"large pore" was drilled through the center of the core along its longaxis. Such a core represents either a zone extreme matrix permeabilityor a fractured zone.

In test 1, the core with the 0.040-inch pore had an initial brinepermeability of about 8200 md. After allowing the brine diluted emulsioncontaining P.O. to contact the core for one hour while the emulsionbroke, a pressure of 20 psi was applied to the core with brine. Theapparent permeability of the core to brine with asphalt in the pore wasabout 76 md. However, the 20 percent diesel oil cutback was extrudedthrough the pore and after flushing the core face with brine and, onflowing brine through the core, the permeability was restored to a5500-md value.

In test 2, H. T. Formaseal (Granular asphalt, Oil Base, Inc.) at aconcentration of one lb/gal was added to the emulsion prior to dilutingit with the P.O. and brine. The particle size of the granular asphaltwas less than 32 mesh (0.0195 inch) so that they would readily bridge onthe pore. The initial brine permeability of the core was 5000 md. Afterthe hydrocarbon phase of the broken emulsion hit the core face, the coreappeared to be completely plugged when 20 psi was applied with brine for20 minutes. No flow was observed in an additional 15 minutes at 40 psiand then 80 psi. Even after the core face was flushed with brine, thecore appeared completely plugged to 80 psi over a period of 20 minutes.However, it is possible that over a longer exposure to brine pressure,the plug would have extruded out of the pore.

In the first run of test 3, the emulsion No. 35-C, containing the harderten percent diesel oil cutback, was used. This emulsion plugged the0.040-inch pore completely at 20 psi for 1.5 hours. The initialpermeability of the core with the large pore drilled through was 5500md. The core remained plugged after increasing the pressure to 40 psifor 15 minutes and then to 80 psi for 20 minutes. After flushing theemulsion off the core face with brine, a pressure of 20 psi finallybroke the emulsion plug. However, the final brine permeability of thecore was only 35 md compared to the initial value of 5500 md. It shouldbe mentioned that the emulsion penetrated into the matrix at the coreface surrounding the large pore and that the final permeability valuewas less than the core permeability without the large pore.

In the second run of test 3, the No. 35-C emulsion was allowed to breakon the core face utilizing the P.O. breaker system. At 20 psi theaqueous phase of the broken emulsion was displaced through thepreviously impaired core at a permeability of about 50 md. When thehydrocarbon phase of the broken emulsion reached the core face, the corewas again completely plugged. After 20 minutes at 20 psi, the pressurewas raised to 40 psi and then 80 psi for another 20 minutes at eachpressure level. The core remained completely plugged by the hydrocarbonphase of the broken emulsion. After the asphalt had been flushed off thecore face with brine, the brine permeability at 20 psi was 13 md. Thisdegree of impairment of the core containing a 0.040-inch diameter poreis taken as a favorable indication of the plugging effectiveness of theemulsion system.

A series of attempts to plug a 0.070-inch pore drilled through one-inchdiameter × 2.00-inch long Berea cores mounted in lucite core holders asin the previous tests. The 120-150 PEN grade asphalt without diesel oiladded was observed to extrude through this size pore at 50 psi. However,with one lb/gal of either H. T. Formaseal or Ameripol No. 1006 (rubbercrumbs, B. F. Goodrich Chemical Company), the 0.070-inch pore wasplugged to 120 psi prior to flushing the core face with brine. In thecase of the tests with H. T. Formaseal, the plug was not obtained withconcentrated emulsion and bridging solids, but was obtained when theemulsion containing H. T. Formaseal was broken by P.O. In the case ofthe test with Ameripol No. 1006 rubber crumbs, the plug was obtainedwith the bridging solids in the concentrated emulsion. Neither of theseplugs was completely effective. The rubber crumb plug extruded throughthe 0.070-inch pore at 120 psi after being shut in overnight. Thegranular asphalt plug was significantly removed by flushing the coreface with brine indicating that the placement of the solids had beenonly near the pore throat opening.

In order to obtain more even dispersion of the granular solids so thatthey would be carried further into the pore, the emulsion wasviscosified with hydroxyethyl cellulose ether (HEC). Guar gum was noteffective in viscosifying the cationic asphalt emulsion. Large clumps ofgel formed in the aqueous phase at a guar gum concentration of onepercent (Basis aqueous phase). An attempt to cross link guar gum in theemulsion with sodium borate at a 0.25 percent concentration (basisaqueous phase) resulted in the emulsion breaking. The Fann viscosityobtained with HEC at 2.5 lbs/bbl in the concentrated asphalt emulsion isshown in Table 3.

                  TABLE 3                                                         ______________________________________                                        PANN VISCOSITY OF CATIONIC ASPHALT                                            EMULSION WITH 65 PERCENT HYDROCARBON                                          PHASE OF 120-150 PEN ASPHALT CONTAINING                                       TEN PERCENT DIESEL OIL WITH OR                                                WITHOUT HEC VISCOSIFIER AT 2.5 LBS/BBL                                        ______________________________________                                                                       rpm                                            Fann Reading                                                                              2      3     100   200   300   600                                ______________________________________                                        Without HEC                                                                               2      3      48   85    130   247                                With HEC   35     54     238   300+  300+  300+                               ______________________________________                                    

In order to prevent extrusion of the soft rubber crumbs while takingadvantage of their pore filling deformability, a 50--50 mixture of therubber crumbs and the granular asphalt was added at a totalconcentration of one-half lb/gal to the HEC viscosified emulsion. Thisemulsion plugged the 0.070-inch pore to 120 psi brine pressure evenafter flushing the core face with brine. The plug was tested atpressures of 20, 40, 80, and 120 psi for one hour at each pressure oneach of three successive days. The plug finally broke at 120 psi on thethird day. It is felt that the performance of this plugging formulationunder laboratory conditions indicates that it should plug rather largefractures under field conditions. Although the P.O. breaker system wasnot used with the viscosified emulsion, the system is compatible andcould be used.

In a similar test utilizing a high pressure Hassler core holder ratherthan the lucite mounted cores, a plug of a 0.04 inch diameter coreformed at a differential pressure of 1000 psi with a viscosified asphaltemulsion containing 1/4 lb/gal Ameripol 1006 rubber crumbs held up tothe following test after flushing the core face with brine.

1 hour at 200 psi -- no flow

1 hour at 400 psi -- no flow

1 hour at 800 psi -- no flow

10 min at 1000 psi -- no flow until plug broke

The typical pH-time curve for the propylene oxide breaker system isshown in FIG. 1. The emulsion contained 63 percent of an MC-800 cutback(150-175 penetration grade asphalt, from Bell Refining Co., Ardmore,Okla.) containing 20 percent kerosene. The emulsifier was a quaternaryamine, Armak E-5 at a concentration of two percent basis total emulsion.In addition, the emulsion contained 0.1 weight percent CaCl₂ for storagestability. This is a chloride ion concentration of about 640 mg/liter.

For the test of FIG. 1, the equivalent three gallons of propylene oxidewas added to one barrel of concentrated emulsion and then this was addedto 2.25 barrels of synthetic brine, the composition of which is given inTable 2. The pH then was determined as a function of time as shown inFIG. 1. The curve is essentially a titration of the cationic emulsifierin the emulsion. The relatively flat portion of the curve, between 10and 30 minutes, shows the buffering effect of the emulsifier. At thepoint at which the pH begins to increase beyond this plateau, theemulsifier has been neutralized by the generated hydroxyl ion and theemulsion breaks. This time is referred to as the initial break time(IBT) for the system. A series of these tests was run at temperaturesfrom 80° F to 140° F and the IBT as a function of temperature is shownas the lower line on a semilog plot in FIG. 2. For this system, the IBTis reduced by one-half for each 13° F increase in temperature. The IBTis quite short at higher temperatures being only about one minute at140° F.

Increasing the P.O. concentration reduces the IBT, as might be expected.The two higher solid lines on FIG. 2 are IBT-temperature lines for P.O.concentrations of two gals/bbl and 1.5 gal/bbl. At 80° F, the IBT withtwo Gals/bbl P.O. was about 60 minutes. However, at 1.5 gal/bbl P.O.,the emulsion did not break at temperatures below 110° F evidently due tothe fact that the P.O. was depleted by the hydrolysis reaction beforethe pH could increase sufficiently to break the emulsion.

What is claimed is:
 1. An oil-in-water emulsion for penetrating into andplugging the pores within the first few inches of a portion of asubterranean earth formation that is encountered by a fluid injectedinto a well, which emulsion consists essentially of:an emulsifiedmixture of an asphalt-containing oil-phase liquid, an aqueous liquid,and a cationic emulsifier, having compositions and concentrations suchthat (a) the particles dispersed within the aqueous liquid are capableof flowing through the pores of said earth formation, (b) the emulsionhas a relatively neutral pH of from about 4 to 8, and (c) the emulsionbreaks when its pH is increased by a significant amount; and dissolvedin said aqueous liquid, a pH-increasing reactant having a compositionand concentration such that the pH of the emulsion is increased to oneat which the emulsion breaks when it is subjected to a time-temperatureexposure of from about 5-60 minutes at from about 80°-140° E equivalentto that to which a fluid is exposed when it is injected into said earthformation, said pH-increasing reactant being a member of the groupconsisting of: mixtures of water-soluble epoxides and water-solublechlorides, mixtures of water-soluble nitrite salts and water-solubleamides of carbonic acid, and water-soluble cyanate salts.
 2. Theemulsion of claim 1 in which said asphalt-containing oil phase liquidcomprises an asphalt of penetration grade of from about 40 to 300 andsoftening point of from about 95° to 145° F cutback with enough dieseloil to provide a softening point near but less than the temperature ofsaid earth formation.
 3. The emulsion of claim 1 in which said cationicemulsifier is a surface active quaternary ammonium salt that forms anemulsion that is stable at a pH of from about 4 to 8.